Direct current arc having reflector means superimposing a reverse complementary image on the arc



Dec. 9, 1952 E GRETENER 2,621,284

DIRECT CURRENT ARC HAVING REFLECTOR MEANS SUPERIMPOSING A REVERSECOMPLEMENTARY IMAGE ON THE ARC Filed July 14, 1947 I 1 lC/mm INVENTOREDGAR GRETENER BY M J V20 WAGE ANGLE OF VIEW ATTORNEYS ARC Patented Dec.9, 1952 DIRECT CURRENT ARC HAVING REFLECTOR MEANS SUPERIMPOSING AREVERSE COM- PLEMENTARY IMAGE ON THE ARC Edgar Gretener, Zurich,Switzerland Application July 14, 1947, Serial No. 760,920 In SwitzerlandJuly 13, 1946 8 Claims.

(Granted under the provisions of sec. 14, act of March 2, 1927; 357 0.G. 5)

The present invention is concerned with increasing and improving theillumination obtainable from an arc source.

It is an object of the invention to compensate for the dark areanormally surrounding the negative electrode of a carbon arc.

It is an object of the invention to distribute the incandescent gasesconstituting a carbon arc in such a manner that the light produced ismaximized.

It is an object of the invention to superimpose upon an arc lampreflector a reverse complementary image of the are so that the lightlevel of the negative dark area is brought up to that of the rest of thearc.

It is an object of the invention to make a carbon are light sourcehaving a brilliant positive crater, a dark negative electrode area and aresultant irregular light distribution or gradient between electrodesinto a light ource which is maximum in both brilliance and homogeneityby first moving some of the glowing ionized vapors forming the are awayfrom the brilliant positive arc crater toward the dark negativeelectrode in such a manner that although the negative electrode area isstill darker than the positive electrode area, the gradient or rate ofchange of light intensity or value between electrodes is oftennonuniform but is usually and in the same direction, that is, constantlydecreasing from the positive electrode to the negative electrode. Whensuch an arc with a variable gradient of light level is combined with itsreversed substantially complementary image not only is the light levelthroughout the arc the same, i. e., substantially zero gradient, but thelight level is maximized because all practically available light isutilized.

Other objects will appear as the description continues. The followingdisclosed structure is intended to be illustrative of a few of the formsthe invention may take and is not to be considered as limiting. In thedrawing like numbers refer to like parts throughout.

In the drawings:

Fig. l is a schematic elevation of a carbon arc with means to distributethe arc.

Fig. 2 is a schematic sectional View of means for superimposing acomplementary arc image.

Fig. 3 is a schematic sectional View of a second arrangement similar toFig. 2 but with a main reflector having a smaller radius of curvature,for increasing the apparent homogeneity of the arc.

Fig. 4 is a schematic sectional View of a third arrangement forincreasing the apparent homogeneity of the arc.

Fig. 5 is a graph of the illumination level measured along the arccolumn of Fig. 3.

Positive carbon I0 and negative carbon I I have an arc I2 ofincandescent carbon gas there between maintained by current flow. Thebright spot'of the normal carbon arc is the crater I3 in the end of thepositive carbon I0, the area around the end of negative carbon II beingrelatively dark. A first step in improving the are as a light source isto move some of the hot gases from around crater I3 to the area aroundnegative corbon II. This may be accomplished by a nozzle I4 aroundpositive carbon I0. Nozzle I4 may be made of metal or ceramic materialas desired and mounted on the carbon holder together with the carbonfeeder usual in carbon arc lamps. Nozzle I4 is shown in outline only andis connected to a supply line I5 through which a gas such as air isforced under pressure. If desired a gas such as nitrogen or carbondioxide may be used to decrease the oxidation of carbons I0 and I I.Where the atmosphere of the arc itself is drawn off and recycled ahigher arc efliciency is maintained. The gas is nozzled to obtainsufficient velocity and caused to flow through annular orifice I6surrounding positive carbon II]. As shown in Fig. 1 the arc I2 issmoothed into a cylindrical column by the flow of the nozzled gas andconcentrated between the electrodes I0 and I I. Parts of the arc I2 arecaused to surround negative carbon II as at IT. This distribution of theincandescent gas of the arc gives a more uniform cylindrical lightsource.

While the above redistribution of the arc I2 aids in forming asatisfactory light source, the arc portion I! is still relatively dark.

This is the first step. The pressure and volume of the air flow aroundarc I2 is so chosen that the distribution of the incandescent ionizedgas yields a more uniform light source cylinder and in so far as ispossible one in which the respective halves or selected portions arereasonably complementary. That is, although the light level along thearc is not uniform the critical peak at the crater has been ironed outand the dark area around the negative electrode filled in so that theblown arc plus its reversed complementary image yields a composite lightsource of uniform intensity along its length. This is true because if toeach instantaneous value of light intensity along the arc is added itsadjusted complementary value the result will be a constant value oflight intensity along the arc. This of course assumes that the pressureand air volume are adjusted so that the arc portions are in factcomplementary or reasonably so over a desired distance.

In carbon arc illumination system it is customary to use a reflectorsuch as a concave mirror I8. The axis of the arc stream I2 can be placedin line with, or perpendicular to, the optical axis of concave mirrorI8. The mirror I8 can reflect only a part of the light from are I2 andalthough there is direct emanation that portion of the light notutilized by main reflector I8 is not efficiently directed. It has beenfound that by placing an auxiliary mirror or reflector iii so as toembrace the solid angle not included by the main reflector 58, areversed image of the light source is superimposed on itself. Mainreflector 18 now reflects both the light falling directly upon it fromarc l2 and that reflected upon it by auxiliary reflector 19. Thus thearc l2, as seen from the main reflector I8, is a light cylinder with anapproximately homogeneous brightness. The lumen output per unit ofapparent arc cylinder area is a substantial constant within reasonablelimits. With this arrangement, it appears to main reflector l8 as if are12 were burning between two positive carbons. By superimposing the imageon to the actual arcv to give a homogeneous brightness distributionalong the arc cylinder axis, the average apparent brightness of the arcstream [2 is considerably increased. More usable light is obtained.

In general, the solid angle of the auxiliary mirror symmetrically placedabout the mid-point of the light cylinder is made equal to the solidangle of the main reflector. The illumination system of the combinedauxiliary mirror and main reflector gives. its maximum li ht o"=tp twhen the total angle of embrace of both reflectors with respect to arci2 is 360 degrees. Such an arrang ment for the case of a parabolic :mainreflector i8 is shown in Fig. 2 in which main reflector l8 subtends 180degrees or one pi radians of the angle about the axis of carbonelectrode i2 and auxiliary reflector I9 also subtends 180 degrees or onepiradians, making 360 degrees in all.

The auxiliary mirror may be used in a number of ways. For example, inFig. 3 the axis 20 of arc stream I2 is perpendicular to optical axis 21of main reflector 22. In Fig. 4 axes 20 and 2! coincide. The are I2 issuperimposed on itself by auxiliary mirror l9 when the center ofcurvature 23 coincides with the mid-point between the ends of carbons Hiand II, which are then imaged at 24 and 25 respectively. From the viewpoint of the main reflector or mirror 22 a practically constant arcbrightness appears between the crater 13 in positive carbon [0 and itsimage 24. Be.- yond these limits the level of illumination rapidlydecreases.

As shown in Fig. 3 where great homogeneity of light'level is desired thefocus 23 is displaced from the midpoint between electrodes toward thepositive crater [3. This brings the reversed image 24 of crater l3closer and makes a shorter effective combined arc image with some lossof light, but with a gain in homogeneity required by high fidelityreproduction with a photo electric cell.

It has been found that auxiliary mirror l9 becomes very hot when placedin close proximity to the are 12 as the radiant energy it receives,While no more than that received by the main reflector 22, isconcentrated in a relatively small mass with a limited radiatingsurface. This is true even though mirror i9 is an excellent reflector.While ceramic reflectors may be used a metal mirror is preferred. First,the metal itself can be polished to reflect. Where a plated reflectingcoating is used the bond between the reflecting coat and the supportingmetal is usually so good that they act as a single piece of metal. Thisis seldom true of metal coatings on ceramic mountings where hightemperatures are involved. Second, metal reflectors are much morereadily cooled. The rapid conduction of heat to all parts permits directwater cooling for highest efficiency. In general, however, forced aircooling is suiflcient. The

back of a metal mirror acts as a suitable radiator of the absorbed heat.It may be finned as at 26 if desired, care being exercised that the fins26 do not intercept the light.

Fig. 5 shows the illumination level with respect to position along thearc column of an arrangement such as shown in Fig. 3. In the graph theabscissa represents distance along the arc between the electrodes shownand the ordinates corresponding values of light level or brilliancy asdetermined by a sensitive light meter. The line of observation duringthe measurements was inclined by an angle of approximately sixty degreesto the axis of the arc column as indicated by arrow ml. The curve )2shows the brightness of the are column without an homogenizing mirror22. Curve I93 shows the increase in brilliancy obtained by superimposingthe inverse arc image on curve H32 and adding the two ordinates. CurveHi l shows the brilliancy of the arc image taken alone. Due to parallaxresulting from the angle of observation the maximum brilliancy or peakof curve I82 will be seen to appear displaced somewhat to the right awayfrom the edge of the crater. The brilliancy of the crater adds to thebrilliancy of the arc column because of the search light eifect of theincandescent hollow crater with its polar axis coinciding with thelongitudinal axis of the arc column. The maximum brlnizancy of thereversed image is somewhat lower than the arc column maximum due to theabsorption losses within the ionized gaseous column and to the deviationfrom unity of the in.- dex of reflection of the reversing mirror. CurveHi2 shows that the brilliancy of the homogenized arc column ispractically or substantially uniform up to a certain point whichcorresponds tothe location of the image of the separation edge betweenthe arc column andthe positive crater. Beyond that critical point, asshown by graph I03, 17119 lrluflllllafilfill ISV'SJ. O1 brnnancylfipluiy cecreases.

It will be noted that the invention achieves its result as a practicalmatter. That is to say for practical purposes graphs Hi2 and I04 arecomplementary in suiflcient degree to combine to produce a substantiallyor practically uniform portion in curve I03. Mathematical exactness inthe sense of a curve portion parallel to the abscissa is not required,but of course the result improves as it is approached.

While I have described my invention in terms of physical structure, itwill be readily understood that structure other than that described mayembody the spirit of the invention.v The claims are presented to setforth the invention in more general terms and the foregoing descriptionis offered as an aid in their interpretation and is not to be taken tolimit their scope. The detailed structures disclosed above are oiferedas specific examples or species of the generic terms appearing in theclaims.

An example of the above is the use of a magnetic focusing coil 27 tocompress the hot ion cloud around crater l3 and move the incandescentgas toward negative electrode H. Again electrostatic fields may beapplied to the arc stream l2 to accelerate ions in their flow and causethe stream 12 to take a more uniform cylindrical shape.

It will be appreciated with the use of coincident axes 20, 2| as shownin Fig. 4-. a blown are or its magnetic equivalent obtained with a coil21 is necessary. A polar diagram of illumination from the commonpositive crater indicates no light from the brilliant crater passes theplane normal to the electrode at the crater end. The threedimensionallight level diagram of the crater forms an oblate spheroid tangent tothe crater l3.

This spheroid is directed away from reflector 22. On the other hand ablown arc produces an intensity diagram in the form of a torussurrounding the arc cylinder and having a substantial portion directedtoward reflector 22 positioned a in Fig. 4.

I claim:

1. In combination in a direct current are lamp assembly, a positivecarbon electrode having a brilliant crater, a negative electrode, meansto move portions of the ionized gas forming the arc away from saidcrater and toward said negative electrode to form a corrected arc columnhaving a light distribution over an effective portion of the distancebetween said electrodes of such value that for each point of saideffective portion the sum of the light value of a given point of saidportion plus the light value superimposed by a corresponding point ofthe reversed image of said portion is substantially a constant for allsuch pairs of points when superimposed in additive relation, a mainreflector for said light producing corrected arc column having a, focalpoint adjacent said column, a spherical auxiliary reflector embracing alarge solid angle of the order of pi radians and having a focal pointpositioned to reflect a reversed image of said corrected arc column uponsaid column, said main reflector serving to project a beam of brillianthomogeneous light combining the beams from said corrected column andsaid reversed image, whereby a carbon are having an irregular lightgradient is given a light gradient of such character over an effectiveportion of the distance between electrodes so that when combined withits complementary reversed image the combination produces a beam oflight having substantially the same level of intensity along a lengthcorresponding to said portion suitable to be condensed to produce anintense homogenous image of its source comprising the corrected portionof the arc column and its reversed image in complementary relation.

2. The combination set forth in claim 1, said means to move portions ofthe ionized gas forming the arc comprising nozzle means surrounding saidpositive electrode to direct an annular stream of gas around and alongsaid positive electrode and the are between said electrodes to form ashort, smooth, brilliant arc-light source cylinder between saidelectrodes.

3. The combination set forth in claim 2, having means to cool saidauxiliary reflector to retain its accuracy as an optical surface.

4. The combination set forth in claim 3, said means to cool saidauxiliary reflector comprising fins on the side thereof removed fromsaid arc and serving both to increase the radiating capacity of the saidauxiliary reflector and the structural strength thereof to minimizewarping.

5. The combination set forth in claim 1, the optical axis of saidreflectors being normal to the axis of said electrodes.

6. The combination set forth in claim 1, the optical axis of saidreflectors being coincident with the axis of said electrodes.

7. In combination in a direct current arc lamp assembly, a positivecarbon electrode having a brilliant crater, a negative electrode, meansto move portions of the ionized gas forming the are away from saidcrater and toward said negative electrode to form a corrected arc columnhaving a light level distribution along an effective portion such thatwhen added to the reversed image of said portion the composite sourcehas a substantialy uniform light level, a main reflector for said lightproducing corrected arc column having a focal point closely adjacentsaid portion, a spherical auxiliary mirror embracing a solid angle ofthe order of pi radians having a focal point also closely adjacent saidportion and positioned to reflect a reversed image of said corrected arccolumn upon said column adjacent the said focal point of said mainreflector, said main reflector serving to project a beam of brillianthomogeneous light combining the beams from said corrected column andsaid reversed image, whereby a carbon are having an irregular lightgradient is given a substantially uniform light gradient betweenelectrodes and combined with its reversed image to produce a beam oflight having substantially the same level of light in tensity throughoutand suitable to be condensed to produce an intense homogeneous image ofits source ocmprising the corrected arc column and its reversed image incomplementary relation.

3. In combination in a direct current are assembly, a positive carbonelectrode having a brilliant crater, a negtaive electrode, saidelectrodes being constructed and arranged to form an arc therebetween, amain reflector for said arc, a spherical auxiliary reflector embracing alarge solid angle of substantially one pi radians having a focal pointpositioned to reflect a reversed image of said are upon said are, saidmain reflector serving to project a beam of brilliant homogeneous lightcombining the beams of said are and its reversed image, whereby a carbonare having an unequal light level distribution along an effectiveportion is combined with a reversed complementary image of said are toproduce a uniform lighting effect.

EDGAR GRETENER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,078,913 Fleming et al Nov. 18,1913 1,853,533 Arbuckle Apr. 12, 1932 2,003,675 Berg June 4, 19352,068,795 Gleick Jan. 26, 1937 2,078,639 Schneider Apr. 27, 19372,107,148 Gretener Feb. 1, 1938 2,204,079 Gelb June 11, 1940 FOREIGNPATENTS Number Country Date 311,704 England Feb. 13, 1930 700,077 FranceDec. 22, 1930

